Steel plate lifting clamps



June 15, 1965 E. M. GARDNER 3,189,377

STEEL PLATE LIFTING' CLAMPS Filed May 21, 1962 2 Sheets-Sheet 1 lllmm 'H56 64 I I H umll I l 6 5" 68 I n 26 I :36 1 1 [I 5 I, I I {1| n i I 70 II In 5 I F 30 I g 78 1 90 7s 72 I8 I I I" "H 74 24 32 28 3e 82 I6 24INVENTOR.

Edward Merrill Gardner ATTORNIEYS June 15, 1965 EwMJGARDNER I 3,189,377STEEL PLATE LIFTING CLAMPS Filed May 21, 1962 2 Sheets-Sheet 2 FIG. 3

INVENTOR Edward Merrill Gardner ATTORNEYS United States Patent YorkFiled May 21,1962, Ser. No. 196,194 2 Claims. (Cl. 24-1ll1) My inventiondescribed and claimed in this application relates to improvements ingripping and lifting clamps of the type used for gripping and liftingheavy steel plates or other heavy structures.

This application is a continuation-in-part of my pending applicationSerial No. 148,623, filed October 30, 1961, which has become abandoned,for Steel Plate Lifting Clamp.

Known lifting clamps include a body having spaced opposed depending jawsbetween which a plate to be lifted is received and gripped between aserrated gripping cam and a fixed serrated pad respectively mounted inthe jaws facing each other. The gripping surface of the cam is cut on anarc and usually coupled by a linkage with the liftshackle of the clampfor the purpose of applying gripping pressure to the cam. Lifting clampsof this or other type sometimes include springs connected with thelinkage or gripping cam for biasing the grioping cam toward the plate tobe lifted. Clamps including a lifting linkage con nected to the grippingcam are usually quite suitable for lifting ordinary plates when thethickness range is small and the plates are not regarded as heavy.

However, such clamps are not entirely satisfactory because it isdifficult to get a full grip on plates when lifting from the horizontaland when there is a wide range of grip on a single clamp, and it isdifficult to manipulate the clamp where the gripping pressure applied tothe gripping cam depends on a linkage connected to a lifting shackle oron a li ht spring biasing the gripping cam. Also on heavy plates theproblem is exaggerated because the linkage exerts ununiform leveragecausing danger of slippage on one thickness and excessive jamming onanother which requires overweight clamp bodies and causes releasingdifficulties on the latter size plates. A constant angle with no linkagesuch as in the clamp of my invention can be controlled to give the exactconditions for any requirements on varying thicknesses of plates in asingle clamp, with or without teeth, on soft or hard, smooth or rough,red hot or cold plates.

Having in mind the structures and deficiencies of the known types oflifting clamps, the primary object of my invention is to provide a clampadapted to grip and safely lift steel plates or steel structures varyingwidely in thickness or texture and weighing up to or 75 tons.

Another very important object of my invention is the provision of alifting clamp which has a simple relatively light weight structure andwhich is easy to operate, and inexpensive to produce.

Another very important object is to satisfy safety engineers by makingthe clamp inoperable unless the cam has had its actuating spring swunginto its operating and locked position. When teeth are new and sharp andcam angle is high in prior clamps, operators get away without theinconvenience of pulling over a heavy spring, but later when dull, theplate will drop. My clamp cannot be used without throwing over the leverto apply the spring and operate the cam.

A further object of my invention is to provide a lifting clamp having astructure so that one clamp can be applied to horizontal plates of awide variation in thickness without rocking and slipping, so that suchplates may be gripped fully up in the throat and be safely lifted andhandled.

According to my invention the improved lifting clamp for gripping andlifting steel plates and other structures varying in thickness fromapproximately 2 inches or more and weighing as much as 10 or tons as asingle unit comprises a clamp body structure having spaced opposedportions or jaws providing a plate-receiving space, open at theprojecting ends of the jaws, a lifting shackle located in the upper partof the body of the clamp above said space, a serrated or smooth grippingpad mounted in the face of one of the depending portions or jaws of theclamp facing the other depending portion or jaw across said space andfreely slidable along the face of the jaw in which it is mounted, agripping cam pivotally mounted in the other depending portion or jaw ofthe clamp operable independently of the lifting shackle, said grippingcam having an arcuate serrated or relatively smooth plategrippingsurface facing toward the gripping pad, and a single spring mechanismpivoted in the body of the clamp and connected to the gripping cam whichmust either force the gripping cam against a plate or must hold it awayfrom a plate engaging position, since there is no neutral position whereit can repose.

In a preferred construction the arcuate surface of the cam is providedwith an angle of contact with the plate to be lifted which is so acutethat an extremely high gripping pressure is exerted by the cam on theplates when the plates are lifted, this pressure which is uniformlyrelated to the weight lifted and not affected by the plate thickness isso high as to imbed the serrations of the cam and gripping padrespectively in the opposite surfaces of the plate without the aid of asupplementary mechanical advantage. Usually the gripping came. ofconventional lifting clamps are cut on the arc of one or two circles,while the serrated arcuate surface of the cam according to my inventionis provided with an arcuate gripping surface having a nearly constantlow angle of contact with the plate to be lifted regardless of thethickness of the plate. Clamps made according to the present inventionfor lifting plates varying in thickness up to 2 inches or more andweighing up to 10 or more tons have been found to hold their grip on theplate even with all of the cam and pad teeth worn olf or ground offrelatively smooth.

In the preferred construction referred to, it will be un derstood thatthe heavy gripping pressure applied, be cause of the low angle of camcontact, is such as to jam the plate between the teeth of the cam andthose of the gripping pad. Therefore, the preferred constructionincludes the freely slidable gripping pad located in an elongated recessin the face of the jaw in which it is mounted. When the clamp is appliedto a plate, the pad may be placed inwardly from its outer position sothat when lifting starts, the pad slides outwardly allowing the cam tohold most of the weight which rotates it and causes the grip. The weightis held by the cam and partially by friction on the back of the padagainst the recess. When the pad reaches the end of the recess and sincethe pad teeth have imbedded into the plate, the pad then holds moreWeight and resists further cam rotation and excessive binding. When alifted plate is brought to rest on the ground, or on some supportingstructure, the pad slides inwardly relative to the body of the clamp asthe latter is moved down, primarily by the stress in the clamp actingdownwardly though the cam, thereby rotating the gripping cam toward theopen position and breaking the jammed position on the plate to give arelaxed contact.

When the gripping cam is rotated and its grip broken in this Way, it isonly necessary to further rotate the cam to its retracted position.Accordingly, the improved clamp is provided with a hand-actuatedoperating mechanism including a strong tension spring which pivots thecam to its retracted position. The structure of this mechanism PatentedJune 15, 1965 is such that one end of the spring is connected to theheel of the cam, while the other end is connected to one end of a leverarm. The other end of the lever arm is pivoted to the clamp body in aposition to provide an overcenter arrangement so that the end of thespring connected to the lever arm may be rotated over a dead centerposition which will snap the arm in between the plates of the frame toboth positions of either on or off. When off, it will lift the cam toits retracted position. When the end of the spring attached to the leverarm is swung to the opposite position, it biases the cam to the plateengaging position. It cannot come to rest on dead center position.

.Since the cam is operable independent of the lifting shackle and itsoperation is not interfered with, the spring arrangement has theadvantage of holding the cam in contact with the plate at all timesduring the handling of the plate and prevents disengagement of the cameven when the plate is lowered to the ground during a handlingoperation. Another advantage is that the spring, which has a strongtension, gives an initial bite on the plate to be lifted before theplate is actually lifted by the crane from the lifting shackle. Ifthrough some malfunction the cam is jammed on a plate and the clamp isto be removed. the spring in opened position gives an opening torque tothe cam so that the instant the cam teeth are jarred loose from theplate, the cam will open fully. This will allow it to be lifted off andplaced on another plate. Furthermore, the operator is always able totell when the grip of the cam on the plate has been released because thespring actually pops the cam to open position,

' In its preferred construction the improved lifting clamp is providedwith a slope at the inner end of the space between'the jaws directedgradually outwardly and toward the jaw in which the gripping cam ismounted. It was found that in using the clamp to lift plates from thehorizontal position, the clamp after being applied to the plate had atendency to tip or roll on the pad because of the weight of the body ofthe clamp and shackle at the side of the plate. This tipping or rollingaction resulted in the clamp being pulled partly oil the plate causingan unsafe condition. However, because of the sloping inner end spacebetween the jaws, it was found possible to push the clamp onto ahorizontal plate, so that its upper inner edge engaged the slope andprevented tipping. This feature is particularly advantageous inconnection with the lifting of plates which are substantially thinnerthan the thickest plates acceptable by the clamp.

The features of the invention are described more in detail in connectionwith an embodiment of the invention as shown in the accompanyingdrawings forming a part of this application.

In the drawings:

FIG. 1 is an elevational view partly in section of a lifting clamp forlifting heavy steel plates shown in engaged gripping position withrespect to a horizontal steel plate to be lifted. The clamp is a weldedsteel structure including steel side plates, but in this view one of theside plates of the clamp is removed in order to reveal the interiormechanism;

FIG. 2 is an elevational view of the top edge of the clamp shown in FIG.1;

FIG. 3 is an elevational view of the clamp shown in FIG. 1, showing theclamp in upright position with the gripping cam and its operatingmechanism in open or released position; and

FIG. 4 is a broken sectional view taken on the line 4-4 of FIG. 1. l

' Referring to FIGS. 1 and 2 of the drawings, the lifting clamp, asillustrated therein, comprises a clamp body provided with fixed jaws 12and 14 spaced apart and facing each other to provide an opening or space15 therebetween for receiving heavy steel plates, for example the plate16 shown in the fully-gripped position with a lifting force applied.

The body lb of the clamp includes a pair of spaced steel side plates 18and 2d of similar contour held in spaced relation with respect to eachother by a thick inverted L-shaped steel spacer block 22 which extendsentirely through the jaw 12 and across the clamp beyond the closed endof the plate-receiving opening 15. The plates 18 and Zil are secured tothe block 22 and held together by rivets 24 and also by peripheralwelding, as indicated. In some cases they are drop forged in twointegral parts with the division through the center of 22, when nowelding is needed. At the projecting end of the jaw 14, the plates areheld in the same spaced relation by a spacer sleeve 26, while the platesat that point are held together by a rivet 28 extending through thesleeve.

At the lifting side of the clamp the plates 18 and extend beyond thespacer block 22 and provide means for securing a heavy lifting shackle30 which is pivoted between the platcs on a shackle pin 32 extendingthrough the plates and provided with a nut secured by a cotter pin. Theportions of the plates 1-8 and 2t) forming the jaw 14 house a grippingcam 34 having an arcuate serrated camshaped gripping edge surface 36, asillustrated. The cam 34 includes an enlarged hub portion 37 having apivot bearing opening through which a pivot pin 38 extends for pivotingthe cam between the side plates 18 and 20. The pin 38 is locatedconsiderably back from the plate-receiving opening i5 and about midwayalong the jaws 14. The pin 38 extends through side plates 18 and 29 onthe respective sides of the cam hub 37 and is secured in place by a nutfastened by a cotter pin.

A gripping pad 42 having a serrated face, facing toward the cam 34 ismounted in an elongated oval-shaped recess 44 cut in the inner face ofthe block 22. The pad 42 includes a large back surface area and asmaller axiallyextending shank 46 extending through an oval-shapedopening 48, the projecting end of the shank being provided with threadsand secured by a nut 54 held from turning by a cotter pin. The nut 50and the threaded end of the shank 46 are located between the plates 13and 20 in a recess 52 in the back of the block 22. The serrations on thepad 42 are preferably concentric sharp projections parallel to the facesof the jaws, as illustrated, and adapted to bite into the steel platebeing lifted, as shown in FIG. 1. The pad 42 is mounted in the jaw 12 sothat it is freely slidable longitudinally in the oval-shaped recess 44,the function and operation of which is explained hereinafter indescribing the operation of the clamp as a whole.

The body of the clamp at the inner end of the opening 15 between thejaws is provided with a sloping surface 53 extending from a selectedposition of the inner end of the opening 15 outwardly and toward the jaw14 housing the cam 34. The angle of the slope as illustrated in FIG. 1is approximately 60 with respect to the face of the jaw 12', or theplate, and it may extend entirely across the opening 15. In any case itmust extend from a position such that the corner edge nearest the cam ofa plate of minimum thickness for the clamp, i.e. the corner along theedge toward the jaw 14 engages the slope 53 and is the only part of theplate engaging the end of the opening 15.

If it is assumed that the plate 16 in FIG. 1 has a minimum thickness forwhich the clamp with the particular cam 34 is adapted, it will be notedthat the edge along the plate 16 in the dotted line position in FIG. 1is the only part of the plate engaging the inner end of the opening 15.The sloping structure is particularly important and useful in applyingthe clamp for lifting plates from the horizontal or nearly horizontalposition, as explained hereinafter in connection with the illustrationin FIG. 1.

The cam 34 is biased and operated by a mechanism, shown at the top inFIG. 1 and in FIG. 2, comprising an overcenter mechanism including astrong spring 54 acting on the cam. In the mechanism, as illustrated,the ends of the spring 54 are respectively wound around and secured tocone-shaped attachment members 56 and 58, the latter of which is pivotedbetween a pair of spaced projecting lugs 69 on a pivot pin e2 (FIG. 2).The pivot pin 62 is inserted from the outside before the cam is mountedin the body of the clamp and extends only through the inside lug. Thelugs 6% are integral with the hub of the cam and located to the sideadjacent the plate 20 and on the opposite side of the pin 38 from theengagement edge 36 of the cam. The member 56 is pivotally mounted on apin 64, having a threaded projecting end, and a nut 66, retained by acotter pin. The nut on the projecting end of the pin 64 retains themember 56 in place adjacent to a spacer sleeve 68 which is welded to theouter movable end of an arm 7%. The pin in extends through the sleeve68, the end of the arm 70, a curled friction spring washer 72 and oneend of an operating handle 74. The projecting upper end of the pin 64-is threaded and carries a nut 76 held in place by a cotter pin.

The arm 7 ti comprises a relatively long rigid steel bar, one end ofwhich carries the sleeve 68 and pin or, While the other end is pivotedto the side plate 18 in slightly spaced relation to the cam hub 37 bymeans of a stud pin '78 extending through a hole in the end of the arm76 and a spacer Washer 89 and which is threaded into the side plate 18.The stud 78 is prevented from rotation by a Dutchman type key 82. Thecam operating and biasing mechanism described above is protected againstdamage by being arranged Within the confines of the side plates 13 and2t), and for this purpose, the inner corners of these plates are cutaway by arcuate bevel type cut-outs $4 and 36.

The handle 74 in the position shown in FIG. 2 lies along the arm 7t)partly in the cut-out recess 84 of the plate 18. The handle, however,when in use is swung outwardly on th pivot pin 64 as shown in FIG. 3. Inits folded position the handle 74 is shown in FIG. 2 as retained by aspringpushed detent or pin ht which snaps into a recess in the handle 74when the handle is moved to the folded position, but this detent may beomitted and the curled spring washer '72 relied on for that purpose. Thehandl 74 as shown in FIG. 3 may be folded-up in like manner. The handleis provided with holes 92 to be engaged by a hook and chain for remoteoperation.

The clamp as described above is particularly useful for lifting andhandling heavy steel plates weighing from to 75 tons and having athickness range in a single clamp of approximately 2 or more inches. Theparts of the clamp are, therefore, constructed and arranged inaccordance with these conditions. For example, the gripping face of thecam as is provided with a contour such as to give a substantiallyconstant angle of contact with the plate to be lifted, regardless of ththickness of the plate within the given range. The pivot axis of the cam34 is below the position of the axis of the pad 42, and in FIG. 1 theangle of contact is illustrated as being the angle between twodot-and-dash lines 94 and 96 extending from the pivot axis of the cam,the line 94 extending through the contact position of the cam with theplate 1.6, while the line 96 extends normal to the face of the plate 16or to a plane along the face of the gripping pad 42. The lines 94 and)6, of course, may represent planes extending through the axis of thepin 38. Several of the teeth of the cam 34 engage and imbed into theplate 1'6 at a position opposite that of the pad 42 and the contactposition is taken at the initial point of contact before penetration ofthe teeth into the plate which is approximately midpoint of the seriesof teeth engaging the plate. The angle between the lines or planes 94and 96 is designated as t1 and preferably has a value approximating Thisvalue may vary within a few degrees but is advantageously within therange of from approximately 10 to approximately The smaller angles givea jamming pressure for certain purposes and in the use of non-marringclamps having large area smooth cams and pads, while the larger anglesare used where ease of release is most important and teeth are used.

The full line position of the cam 34 in FIG. 3 is theretracted positionin which the cam is located between the side plates 15 and 2d and heldthere by-the overcenter mechanism, including the spring 54 which isalways under tension. The cam is independent of shackle 30 and,

the spring with the leverage provided on the cam is adapted to lift thecam to the retracted position when the overcenter mechanism is swung tothe position shown in FIG. 3. FIG. 3 also shows the cam in dotted lineposition gripping a four inch plate 16', also shown in dotted lineposition. In this instance the angle of contact a is approximately thesame as that in FIG. 1, approximately 13 to 14. When the angle ofcontact is made so acute that a very high gripping pressure results whenthe plate is lifted, the plate is actually jammed or wedged between thesurfaces of the cam and those of the pad The teeth of the cam and padbeing imbedded are interlocked so that to reverse the cam motion, theteeth on a non-sliding pad would have to displace some metal on theplate.

In the lifting of steel plates from the horizontal or nearly horizontalposition, especially where these plates were somewhat thinner than thewidth of the opening between the jaws of the clamp, it was discoveredthat the clamp, without the sloping surface 53, when applied to theplate would roll on the pad and tip because of the Weight of the shackleand end portion of the clamp extending over the edge of the plate, sothat when a lifting force was applied to the shackle, the gripping camwould roll on the surface of the plate, causing an engagement which wasnot far enough from the edge, thereby impair ing the safety of thelifting operation.

These disadvantages are overcome in the clamp accord ing to the presentinvention by the provision of the sloping surface 53 which in useprevents the tipping or rolling of the clamp on the pad. This isexplained in connection with the operation of the clamp for lifting theplate 16 in accordance with the showing in FIG. 1 of the drawings. Inthis figure the plate 16 and the parts of the clamp are illustrated intheir final full-line position after the horizontal plate is gripped bythe clamp. When applying the.

clamp to the horizontal plate 16, the cam i in the retracted positionillustrated in FIG. 3, and the operator slides the clamp onto the plate16 and then applies the overcenter mechanism to rotate the cam intoengagement with the upper surface of the plate. This has the effect oflifting the body of the clamp relative to the plate to move it to aboutthe relative position shown in FIG. 1.v

After the clamp i in this position, and before any lifting force isapplied to the clamp, the operator slides the clamp farther onto theplate, while keeping the pad up against the underside of the plate,until the corner edge is at the dotted line position in engagement withthe sloping surface 53. This is the only point at which the plateengages the end of the opening 15 in the clamp and the slope is suchthat the plate is caught on the sloping surface 53, thereby preventingthe clamp from rolling on the pad 42 or tipping with respect to theplate 16 so that the inner surfaces of the jaws of the clamp are heldpractically parallel with the plate.

With the clamp held in the position described, the teeth of the cam 34will partially imbed in the surface of the plate because of the pressureof the spring 54, and the same will be true of the serrations in the pad42, and the:

without any slippage, and as the plate is lifted, the clamp.

will move slightly relative to the plate until the plate is at about thefull-line position in FIG. 1 and the pad 42 has slid outwardly in theclamp to the outer end of the recess 44.

It will be understood that any plate having a thickness greater than theplate, 16 and less than the full width of the space 15, when lifted fromthe horizontal position,

will have its upper edge engaging the sloping surface 53, so that notipping or rolling of the clamp occurs while it is being applied to theplate.

The slope of the surface 53 should be such that the edge corner of theplate will not readily slide thereon when the cam and pad engage theplate. The slope illustrated in the drawings is approximately 60 withrespect to the surface of the plate or to the inner surface of the jaw12. This slope may be within the range of from 45 to 75.

If a cam were provided in the clamp illustrated in the drawings whichwould be long enough to be useful for lifting plates as thin as /8 inch,it will be understood that the slope 53 would necessarily extendentirely across the inner end of the opening 15. As illustrated in FIG.1, the plate 16 has about the minimum thickness to be lifted by theclamp as shown and, therefore, the sloping surface 53 is extended only ashort distance below the upper surface of the plate. The sloping surfaceprevents the rolling and tipping of the clamp and riding of the platewhen it is'applied to the plate and, therefore, promotes the safeoperation of the clamp in the lifting of heavy plates of the typereferred to.

If the plate 16 is placed in a vertical position at the point oflocation after being lifted by the clamp, the clamp is lowered relativeto the plate and pad until the edge 55 engages the slope 53, whereuponthe clamp is released in the manner described hereinafter in connectionwith FIG. 3. If the plate is located in a horizontal position afterlifting, the release of liftingpressure on the clamp will permit thetremendous stress and strain in the clamp to effect slightcounterclockwise rotation of the cam 34 and the sliding of the pad 42 inthe recess 44. Then the operator can swing the overcenter mechanism tothe position shown in FIG. 3, whereupon the cam 34 will be retractedinto the body of the clamp so that the clamp is completely released.

Only if extenuating circumstances exist will the cam stick when theovercenter mechanism is moved to the position in FIG. 3, because theinternal stress along line 94 is so great that its component forcedownward is sufficient to slide the clamp body downward on the back ofthe pad 42. Cam 34 can roll in its tracks without metal displacementuntil it is free. Then the pad teeth can move outwardly from its deepteeth impressions. The cam contact angle is critical here, as too highan angle will not grip, but too low an angle will not release, so theproper constant angle is critical.

In the operation and use of the clamp for lifting plates in an uprightposition as shown in FIG. 3, with the lifting shackle 39 attached to acrane or other lifting mechanism, the clamp, with the cam in theretracted position as shown in FIG. 3, is let down over a plate, such asthe 4 inch plate 16', until its left-hand corner engages the slopingsurface 53. When the clamp has been lowered to this position, theoperator, by the use of the handle 74, swings the overcenter mechanismto the position shown in FIGS.

1 and 2, thereby rotating the cam on its pivot pin 38 into engagementwith the plate. The tension of the spring 54- is such as to apply astrong gripping pressure sufficient to slightly imbed the teeth of thecam and the pad into the plate. When the clamp and plate are lifted, theteeth are further imbedded into the plate, a spreading strain is on thejaws and the cam is rotated slightly. This results in the plate slippingon the pad, if it is all the way down, until equilibrium is establishedbetween the stresses and strains in the clamp, and the plate is jammedbetween the cam and pad because of the small acute angle of contact.position shown inFIGS. 1 and 3, that is, at the bottom of the ovalrecess 44. The pad is freely slidably mounted in the jaw 12 so that whenthe clamp is in upright position and not gripping a plate, it is alwaysin the lower position.

7 After the plate 16 is lifted to the desired location or In theseoperations the gripping pad 42 is in they e2 set to rest upright on theground, the clamp is lowered by the crane and the weight of the clampand downward reactive force of the cam on the clamp body carries itdownwardly with respect to the plate and pad, because the pad 42 slidesupwardly in the recess 44 while it is still in engagement with theplate. The clamp moves down and is held by the spring actuated cam whichhas rolled back almost to its point of beginning where the cam is heldonly by the spring in the grooves it made when the load was started tobe applied, thereby loosening the pressure of its teeth in the surfaceof the plate caused by lifting. At this point the operator either byhand or by a hook pulls out the handle 74 and swings the arm 76 andpring 54- around to the position shown in FIG. 3, so that the springtension is applied in a direction to move the cam 34 to the retractedposition shown in FIG. 3. If the plate 16 is let down to a horizontalposition after being lifted, the clamp is released in the mannerdescribed above with respect to FIG. 1.

While the cam 34 has some weight and may be lightly caught in theindentations by the spring pressure, the spring 54 has such a force andis connected at such a distance from the pivot axis of the cam that whenrcversed, it applies a torque to the cam sufficient to actually flip thecam to the retracted position and hold it there. The overcentermechanism comprising the bar or arm 7%) and spring 54 are locatedcompletely within the borders of the plates 18 and it} in eitherposition, so that they cannot be damaged it the clamp is banged around.The handle 74 is also foldable in on the side of the bar 70.

In the operation of a clamp provided with a low cam angle the plates arewedge and jammed in the clamp when lifted because of the small contactangle of the cam. The vertically slidable gripping pad 42, therefore,cooperates with the cam to effectively loosen the jamming grip when theplate is set down and the clamp permitted to move relative to andfarther onto the plate. The pad 42 is provided with a large surfacesurrounding the shank 46, as apparent from the showing in FIG. 4, forexample, so that the back surface of the pad slides readily on the backsurface of the oval cut-out or recess 44.

if during the handling of a plate, such as the plate 16, it is set downmomentarily and the clamp allowed to move downwardly with respect to theplate 16 moving the gripping pad upwardly and rotating the cam, theclamp will not be released from the plate because of the continuingstress applied by the spring 54. There is, therefore, no danger of theclamp losing its grip on the plate because, with the spring 54 and bar7d in the position shown in FIG. 1, the plate may be relifted, and whenit is, the pad 42 merely slides down to its lower position and the camrotates back to its jamming position, with the teeth of both occupyingtheir original positions in the plate.

The safety and ease of operation of the clamp is greatly enhanced by thesloping surface 53 particularly in lifting plates from the horizontalposition. The clamp is also safe because the gripping cam is not linkedto the shackle and no plate can be lifted unless the operator appliesthe clamp, i.e., unless the overcenter mechanism is operated to put thecam in engaging position on a plate. Where in known types of clamps thecam may contact the plate Without the spring pressure being applied, theoperators using such clamps sometimes lift the plates without applyinrthe spring tension against the cam. This is impossible in my arrangementand because of this, some safety engineers have declared it a greatsafety feature.

Since only one illustrative example of a lifting clamp according to theinvention has been described, it is to be understood that certainmodifications and changes may be made without departing from the spiritand scope of the invention as defined by the appended claims.For'example, while the clamp may be made of steel, it is understood thatit may be made of other suitable alloys and that the cam and pad will bemade of hardened steel or other suitable hardened alloys, also that theparts may be made of forgings or castings where some of the parts may becombined into less parts With reinforcing ribs and parting linesaltered.

I claim:

1. In a clamp of the type used for lifting steel plates including a bodyhaving spaced opposed projecting jaws defining a U-shaped structure andproviding a p1ate-re ceiving space between the jaws open at theprojecting ends of the jaws, said space being adapted for receiving asteel plate to be lifted by the clamp, a lifting shackle for the clampopposite said space, a gripping pad carried by one of said jaws andhaving a gripping surface facing the other jaw across said space, and agripping cam having an arcuate plate gripping surface and beingpivotally mounted in the other jaw with its arcuate gripping surfacefacing toward the gripping surface of the pad, the improvement in whichsaid pad is slidably mounted for lengthwise movement relative to theinner face of said one jaw, the inner face of said one jaw including arecess extending lengthwise of said one jaw, said gripping pad includinga head slidably located in said recess and having a serrated portionprojecting from the inner face of said one jaw, said one jaw beingprovided with an opening extending lengthwise of the recess, and a shankon the back of the head of the pad extending into said one jaw andmovable lengthwise in said opening.

2. A clamp as claimed in claim 11, in which the cam is arrangedindependent of the lifting shackle, and an overcenter mechanismcomprising a tension spring one end of which is attached under tensionto the heel of the cam and which in one position forces the arcuatesurface of the cam into gripping engagement with a steel plate locatedin said space and which in another position holds the cam in the jaw inwhich it is mounted.

References Cited by the Examiner UNITED STATES PATENTS 747,()65 12/03Harnann 294-101 2,370,411 2/45 Monaco 294104 2,446,610 8/ 48 Renfroe294-104 2,986,422 5/61 Renfroe 294101 SAMUEL F. COLEMAN, PrimaryExaminer. ANDRES H. NIELSEN, LOUIS J. DEMBO, Examiners.

1. IN A CLAMP OF THE TYPE USED FOR LIFTING STEEL PLATES INCLUDING A BODYHAVING SPACED OPPOSED PROJECTING JAWS DEFINING A U-SHAPED STRUCTURE ANDPROVIDING A PLATE-RECEIVING SPACE BETWEEN THE JAWS OPEN AT THEPROJECTING ENDS OF THE JAWS, SAID SPACE BEING ADAPTED FOR RECEIVING ASTEEL PLATE TO BE LIFTED BY THE CLAMP, A LIFTING SHACKLE FOR THE CLAMPOPPOSITE SAID SPACE, A GRIPPING PAD CARRIED BY ONE OF SAID JAWS ANDHAVING A GRIPPING SURFACE BEING THE OTHER JAW ACROSS SAID SPACE, AND AGRIPPING CAM HAVING AN ARCUATE PLATE GRIPPING SURFACE AND BEINGPIVOTALLY MOUNTED IN THE OTHER JAW WITH ITS ARCUATE GRIPPING SURFACEFACING TOWARD THE GRIPPING SURFACE OF THE PAD, THE IMPROVEMENT IN WHICHSAID PAD IS SLIDABLY MOUNTED FOR LENGTHWISE MOVEMENT RELATIVE TO THEINNER FACE OF SAID ONE JAW, THE INNER FACE OF SAID ONE JAW INCLUDING ARECESS EXTENDING LENGTHWISE OF SAID ONE JAW, SAID GRIPPING PAD INCLUDINGA HEAD SLIDABLY LOCATED IN SAID RECESS AND HAVING A SERRATED PORTIONPROJECTING FROM THE INNER FACE OF SAID ONE JAW, SAID ONE JAW BEINGPROVIDED WITH AN OPENING EXTENDING LENGTHWISE OF THE RECESS, AND A SHANKON THE BACK OF THE HEAD OF THE PAD EXTENDING INTO SAID ONE JAW ANDMOVABLE LENGTHWISE IN SAID OPENING.